The present invention relates generally to optical frequency conversion, and more specifically to a multi-layer gallium arsenide-based Fresnel phase shift optical device for infrared wavelength conversions.
Optical frequency conversion involves fabricating a crystal in which a laser of one frequency is directed through the crystal and a portion of the light converted to another frequency. The conversion process can be understood by considering it as occurring in the visible light range where a portion of a beam of one color, for example, blue, is converted into a beam of another color, for example, red.
There are a number of nonlinear optical materials that have been successfully used for frequency conversion. Very few, however, have been successful in the mid-infrared (IR) optical range. Periodically poled lithium niobate (LiNbO3) is a nonlinear optical material that does work in the mid-IR range. This technology makes use of the ferroelectric property of LiNbO3. An intense electric field is applied to permanently induce a periodic inversion of the crystal structure. This inversion allows for quasi-phase-matching (QPM, a standard nonlinear optics technique for frequency conversion) in that the crystal inversion produces a spatial modulation of the non-linear optical coefficient. However, the transparency of LiNbO3 is low for wavelengths above 4 microns, limiting its use for IR and Terahertz (THz) sources.
Gallium Arsenide (GaAs) is a more promising material because it is transparent up to 17 microns, has a larger effective non-linear optical coefficient, and can be easily processed using existing semiconductor fabrication method. GaAs is not ferroelectric and an inversion of the crystal cannot be achieved by poling.
GaAs crystals have been shown in the prior art to provide efficient optical frequency conversion into the IR frequency range. The prior art teaches a variety of approaches for fabricating GaAs crystals for this purpose.
In one prior art approach, thin flat plates are used in which the end walls are angled to allow light to enter and exit. Once light enters the angled end wall, it reflects at an angle first off the top and then off the bottom of the plate thereby traveling down the plate from one end to the other. The angled exit end wall allows the newly generated light to easily exit. The reflections off the top and the bottom have an important role in the frequency conversion process and, without them, no frequency conversion occurs. The more reflections, the higher the conversion efficiency.
An improvement on the thin plate approach uses quasi-phase-matching by alternating the crystal orientation, resulting in a change in the sign of the non-linear coefficient. QPM GaAs has been demonstrated using stacks of bonded wafers, as described, for example, in E. Lallier, M. Brevignon, and J. Lehoux, Optics Letters, “Efficient Second-Harmonic Generation of a CO2 Laser with a Quasi-Phase-Matched GaAs Crystal,” Vol. 23, pp. 1511-1513 (1998)). There is, however, significant scatter and signal loss associated with the bonded interfaces. QPM structures with periodic reversal of the GaAs orientation have been produced using epitaxial growth on patterned templates, so called Orientation Patterned GaAs (OPGaAs), and IR and THz generation has been observed. This technique, however, requires epitaxial growth of very thick layers (approximately 1 mm) while maintaining the periodically reversed GaAs structure. See, for example, A. Eyres, P. J. Tourreau, T. J. Pinguet, C. B. Ebert, J. S. Harris, M. M. Fejer, L. Becouarn, B. Gerard, and E. Lallier, “All-Epitaxial Fabrication of Thick, Orientation-Patterned GaAs Films for Nonlinear Optical Frequency Conversion,” Appl. Phys. Lett., vol. 79, pp. 904-907 (2001). Propagation of the inverted GaAs domains is not always successful, limiting the efficiency of the conversion process.
The OPGaAs method has trouble growing on thin columns. OPGaAs fabrication is based on a photolithography and molecular beam epitaxy (MBE) process that results in a thin-film template with periodic crystal inversions. These periodic crystal inversions consist of many thin, long columns (5 μm-200 μm wide, 4-5 mm long). A thick film (0.5-1 mm) is then grown upon this template by hydride vapor phase epitaxy to produce bulk OPGaAs. The film thickness must be thick enough to allow the pump laser to pass through the columns from the side. One problem with the technique is that it is nearly impossible to grow thin columns high enough to be useful for nonlinear optical applications. The columns close up before useful thicknesses can be obtained.
A third method for obtaining nonlinear optical conversion in GaAs has been described in J. A. Armstrong, N. Bloembergen, J. Ducuing, P. S. Pershan, Phys. Rev., “Interactions Between Light Waves in a Nonlinear Dialectric,” vol. 127, pp. 1918-1939 (1962); and, R. Haidar, N. Forget, P. Kupecek, E. Rosencher, “Fresnel Phase Matching for Three-Wave Mixing in Isotropic Semiconductors,” J. Opt. Soc. Am. B., vol. 21, pp. 1522-1534 (2004). It is sometimes referred to as Total Internal Reflection Quasi-Phase-Matching (TIR-QPM).
Quasi-phase-matching for this third method is obtained by the use of the Fresnel phase shift that occurs on reflection (bounce) in the non-linear crystal.
One of the disadvantages of the TIR-QPM technique is that the number of phase reversals that can be reasonably produced in GaAs (40-70 bounces) is much less than can be produced in an equivalent OPGaAs structure.
It is, therefore, an object of the invention to provide a new and improved approach for optical frequency conversion using GaAs crystals.